A radiosonde is a device which is carried aloft into the upper air atmosphere by a sounding balloon for the purpose of measuring and humidity and temperature at different heights or barometric pressures reached by the unit in flight. Basically the radiosonde comprises a radio frequency transmitter having a given carrier frequency which is either frequency or amplitude modulated by an associated modulating oscillator whose frequency is determined by sensing elements, whose impedances are responsive to the respective atmospheric conditions they are to measure, which are switched into and out of the control circuit of the oscillator. The carrier frequency throughout most of the world assigned to radiosondes is usually 107 megahertz for weather ships or 403 or, more usually, 1680 megahertz when flown off land.
While radiosondes have developed with the electronics art insofar as the radio frequency and modulating oscillators are concerned, until the present invention it has been found necessary, in order to meet performance requirements to effect switching of the sensing elements into and out of the control circuit of the modulating oscillator by means of an electromagnetic relay or mechanical switch. Such a switch has the important advantage over other previously known switching arrangements of having contacts which have a low impedance which is not affected by the extreme temperature changes to which the switch is subjected. In this connection, it will be appreciated that a radiosonde must be inexpensive to produce since it is only flown once and yet the performance specifications are very tight and the environmental conditions severe. Probably the one most extreme environmental factor is the wide temperature range (+30.degree. C. to -80.degree. C.) that the device must operate under with a negligible effect on its performance. While the impedance characteristics of the electromagnetic relay or mechanical switch contacts are satisfactory, the disadvantages of an electromagnetic relay are cost, power consumption, mass and the susceptibility of its contacts to chatter, stick or burn. The mass of the electromagnetic relay itself and the capacity and size of the associated circuit elements which the relay's power consumption dictates adds to the hazard which present radiosondes present to jet aircraft. The chattering, sticking or burning of the relay contacts affects reliability of the radiosonde and of course the relay's cost is a serious drawback.
In attempting to find a switch to replace the electromagnetic relay the problem encountered has been the variable impedance and hence measuring errors which other switching introduces into the measuring or sensing element circuits particularly under the wide temperature changes, and, until the present invention, no other switching has been found which is acceptable.